.TH UBFA 6.SH NAMEubfa \- universal binary format for data transport.SH DESCRIPTION.I UBF(A)is the data transport encoding for Armstrong'sUniversal Binary Format.It provides four primitive types: atoms (symbolic constants), integers, strings, and binary data.There are two compound types: fixed-length tuples and variable-length lists..IR Ubfa (2)provides basic support in Limbo for reading and writing streams of UBF(A)-encoded data..PPThe.I inputsyntax is defined by the following rules:.IP.EX.ft R.ta \w'\f2simple-xxx\f1'u +\w'\ ::=\ 'u\f2input\fP	::=	\f2item\fP* '$'\f2item\fP	::=	\f2integer\fP | \f2atom\fP | \f2string\fP | \f2binary\fP | \f2tuple\fP | \f2list\fP | \f2store\fP | \f2push\fP | \f2comment\fP | \f2tag\fP \f2integer\fP	::=	\f5'-'\fP?\f5[0-9]\fP+\f2atom\fP	::=	"'" (\f5[^\e']\fP | '\e\e' | "\e'")* "'"\f2string\fP	::=	'"' (\f5[^\e"]\fP | '\e\e' | '\e"')* '"'\f2binary\fP	::=	'~' \f2byte\fP* '~'      # preceded by \f2integer\fP byte count \f2tuple\fP	::=	'{' \f2item\fP* '}'\f2list\fP	::=	'#' (\f2item\fP '&')* \f2store\fP	::=	'>' \f2reg\fP\f2push\fP	::=	\f2reg\fP\f2reg\fP	::=	\f5[^-%"~'`{}#& \en\er\et,0-9]\fP\f2comment\fP	::=	'%' (\f5[^\e%]\fP | '\e\e' | '\e%')* '%'\f2tag\fP	::=	'`' (\f5[^\e`]\fP | '\e\e' | '\e`')* '`'.EE.PD.DT.PPWhite space is any sequence of blank, tab, newline or carriage-return characters, and can appearbefore or after any instance of.I itemin the grammar..PPThe.I inputdata is interpreted by a simple virtual machine.The machine contains a stack of values of primitive and compound types, and a set of registers also containingvalues of those types.White space and comments are ignored.Primitive.IR integer ,.I atomand.IR stringvalues are pushed onto the stack as they are recognised.Certain input bytes outside any value act as operators:.TP.B {Note the current stack depth..TP.B }Pop stack values to restore the most recently noted stack depth.Push a single valuerepresenting a tuple of those items; the left-most value in the tuple is the last one popped(the first in the original input stream)..TP.B ~Pop an integer value.I nfrom the stack.Read.I nbytes from the input stream and push a value onto the stack that represents them.The next byte must be the character.BR ~ ,which is discarded..TP.B #Push a value representing an empty list onto the stack..TP.B &Pop a value.IR v .Pop another value.IR l ,which must represent a list.Push a value that representsthe list.IB v :: l .(Note that the items in a.I listtherefore appear in reverse order in the input stream.).TP.BI > regPop the top value from the stack and store it in a register labelled by the byte.IR reg ..TP.I regPush the value of register.I reg(which must be non-null) onto the stack..TP.I tagAssociate the tag string with the value on top of the stack.The.IR ubfa (2)implementation does so by replacing it by a special.B Tagtuple..TP.B $End-of-input: there must be exactly one value on the stack,which is the result..PPApplications using UBF(A) typically take turns to exchange.I inputvalues on a communication channel..SH SEE ALSO.IR ubfa (2),.IR json (6),.IR sexprs (6).brJ L Armstrong, ``Getting Erlang to talk to the outside world'',.I "ACM SIGPLAN Erlang workshop 2002" ,Pittsburg, PA USA.brUBF web page,.B "http://www.sics.se/~joe/ubf/"